Zimbabwe's opposition leader Nelson Chamisa on Friday lashed the results of the country's landmark presidential elections as "fraudulent" and "illegitimate" but signalled he would take the path of law to contest the outcome.

"We won this election and we are ready to form the next government," Chamisa told a press conference, after President Emmerson Mnangagwa of the ruling ZANU-PF was declared victor with 50.8 percent of the vote.

Chamisa, leading the main opposition Movement for Democratic Change (MDC), won 44.3 percent in the historic first election following the ousting of autocrat Robert Mugabe, according to the Zimbabwe Election Commission (ZEC).

Chamisa charged that there had been flagrant rigging under ZEC, a body which under Mugabe was notorious for fraud.

"Mr Mugabe was at least sophisticated," he complained.

ZEC officials have robustly denied allegations of bias or rigging.

Mnangagwa was named ZANU-PF leader after the brief military intervention which toppled Mugabe in November after 37 years of iron-fisted rule.

Chamisa called the electoral commission "opaque, partisan, arrogant".

"We are not accepting fake results," he said.

"We are not accepting this fiction. We want a proper result to be announced. We will pursue all means necessary, legal, constitutional, to make sure we protect the people's vote."

Chamisa said the MDC would dispute the results through the courts, though a legal challenge appears to offer little hope of overturning the outcome.

He urged supporters to refrain from violence after troops opened fire on demonstrators in Harare on Wednesday, killing six people.

Chamisa's press conference at a hotel in Harare was delayed after riot police armed with tear gas broke it up, clearing journalists out of the building and prompting tense exchanges.

But it went ahead after acting information minister and ZANU-PF spokesman Simon Moyo told the police to stand down.

Zimbabwe's MDC leader Nelson Chamisa addresses a press conference at The Bronte Hotel in Harare, on August 3, 2018